Fix jumping masthead text
Fix jumping masthead text

In cases where a city has more images than strap lines, the masthead text would jump to a different height when transitioning from images with strap lines to those without. This fix keeps the text in place whether `.strapline` is empty or not. I have tested the change against cities with no empty strap lines, continents, and countries. No behavior changes found in desktop or mobile views.

lonelyplanet/rizzo-next,lonelyplanet/rizzo-next